Obtaining a precise understanding of the mechanism(s) by which endogenous neuromodulators such as dopamine (DA) regulate glutamergic excitatory transmission could lead to better for patients afflicted with epilepsy, Parkinson's Disease, schizophrenia, and drug addiction. Activation of the D1 dopamine receptor subtype, in particular, modulates glutamate-induced neuronal excitability in frontal cortex, hippocampus and neostriatum. Little is known about molecular mechanism(s) by which D1 receptor signaling modulates excitatory transmission. We recently identified a single transmembrane protein designated Calcyon which interacts with D1 DA receptors and localizes to vesicles in dendritic spines. Via interaction with Calcyon, D1 receptors stimulate release of Ca++ from internal stores (Ca++i) in an activity-dependent manner in heterologous expression system. A similar activity-dependent, D1 receptor agonist stimulated response is detectable by Fura-2 Ca++ imaging of primary cultures of cortical and hippocampal neurons. Our overarching hypothesis is that Calcyon enhances D1 receptor signaling through Gq/11 by serving as a link connecting D1 receptors to IP3 regulated Ca++ stores. We will determine how D1 DA receptor signaling through Gq/11 is primed, and how Calcyon enhances D1 receptor-mediated IP3 signaling. Using Ca++ imaging and peptides to block the D1 receptor:Calcyon interaction, we will determine if Calcyon is required for D1 agonist stimulated Ca++i release in dendritic spines. We propose to identify other GPCRs and accessory proteins that interact with Calcyon by yeast two-hybrid and phage display screens. We will determine how these proteins are involved in regulating release of Ca++ from vesicular stores. The results of this research should provide insight into clinical strategies to selectively boost or dampen D1 receptor mediated neuromodulation of excitatory transmission.